1. Field of the Invention
This invention relates generally to methods and systems for providing reliable altitude measurements for aircraft. More particularly, the invention relates to redundant altimeter systems for aircraft.
2. Description of Related Art
As air travel has become increasingly commonplace, increasing numbers of aircraft fill the sky, leading to growing concerns about safety and efficiency in the use of available airspace. Aviation authorities, such as the United States Federal Aviation Authority (FAA), have been entrusted with establishing rules for the use of the airspace, and for ensuring that the available airspace is used safely, efficiently and effectively.
Since aircraft are not bound to preset physical routes, such as roads or railways, in the guidance of aircraft along predefined air routes, one of the most important sets of safety rules promulgated by aviation authorities relates to the separation of aircraft. Such traffic separation rules ensure that aircraft which are flying in the same airspace are of the same type and are travelling generally in the same direction. This is intended to avoid, for example, a jumbo jet flying west having to dodge private planes or jets travelling east at the same flight level.
To manage the air corridors efficiently, the FAA has since the 1950""s mandated a Vertical Separation Minimum of 2000 feet at flight levels above 29,000 feet (Flight Level 290). While this has worked well for many years, the available airspace above FL 290 has grown more crowded as increasing numbers of aircraft seek to use the finite airspace that exists between flight level 290 and flight level 600.
To accommodate this growth, beginning in 1997 and continuing through 2002 the FAA is phasing in Reduced Vertical Separation Minimum (RVSM) regulations, which mandate a 1000 foot minimum required vertical separation in certain prime airspace (above FL 290 and ultimately extending to FL 410). This will effectively double the amount of air traffic concurrently permitted in that air space, relieving some of the congestion inherent in the prior system.
With a reduction in vertical separation minima, however, precision in measuring altitude becomes even more crucial to the safety of aircraft. Measurement errors which could heretofore be disregarded when an aircraft""s actual altitude could safely vary by several hundred feet from its measured altitude are unacceptable when the minimum vertical separation is reduced to 1000 feet.
To accommodate the need for greater precision in altitude measurement, new FAA rules (including Interim Guidance Material on the Approval of Operators/Aircraft for RVSM Operations, 91-RVSM, as amended Jun. 30, 1999; and Master Minimum Equipment List Global Change 59, PL-84, Aug. 15, 1997) mandate the use of at least two highly accurate, and independent, altitude measurement systems to ensure that each aircraft operate and remain in the appropriate altitude range for its type and direction of travel. These new rules require that if during the flight one of the altitude measurement systems is deemed inaccurate, the aircraft must leave RVSM airspace and thereafter fly only in non-RVSM airspace.
The RVSM range of altitudes, however, is the prime range in commercially traveled airspace because it includes the most fuel-efficient altitudes. It is therefore highly desirable for aircraft to use RVSM airspace whenever possible. Being required to leave the RVSM airspace can accordingly have dramatic effects on the aircraft""s operating efficiency, mandating the use of more fuel which decreases the attainable range and makes the flight more costly to the operator. On long flights, such as across the Pacific, flying in non-RVSM airspace may increase fuel usage by an amount sufficient to force the aircraft to make an unscheduled landing to re-fuel, thereby delaying the flight. For scheduled commercial flights, this can be disastrous in terms of cost and increased passenger dissatisfaction.
As stated, under the new FAA rules, to fly in RVSM airspace an aircraft must maintain at least two working error-corrected altimeters to provide accurate cross-checked altitude data to the cockpit crew. Altimeters are xe2x80x9cerror-correctedxe2x80x9d when they are calibrated to take into account the dynamic and static pressure variations which occur during altitude measurements.
The FAA permits the certification of groups (i.e. of at least eight) of identically manufactured aircraft with respect to their altitude measurement. To obtain a group certification, the manufacturer must engage in extensive computational fluid dynamics (CFD) calculations, as well as in-flight testing of the group exemplars, including use of a trailing cone for measuring pressure altitude.
The computational portion of the correction may be performed on the ground, based upon the physical configuration of the aircraft and the manufacturing tolerances for imperfections in the external xe2x80x9cskinxe2x80x9d of the aircraft, i.e. the xe2x80x9cwavinessxe2x80x9d of the skin.
In-flight measurement of altitude is based primarily upon monitoring of the static air pressure on the exterior of the aircraft. All other measurements are used to adjust the basic calculation of altitude based upon the static air pressure. There are many sources for error in the calculation of altitude based upon static air pressure, such as possible leaks in the conduits used to covey the air for measurement, mechanical errors in the system, etc. These errors are collectively referred to as static source errors, and altimeters are routinely calibrated to account for these errors with a correction factor known as the Static Source Error Correction (SSEC). Once an aircraft is manufactured, most aspects of the SSEC may be calculated with only minimal in-flight testing. With these tests performed, and a group of aircraft certified, the SSEC for the entire group may be determined within RVSM requirements. The SSEC for an aircraft is maintained in the air data computers (ADCs) of the aircraft.
The SSEC for an aircraft varies however with the operating speed of the aircraft due to variations in the fluid air flow over the aircraft skin, and is therefore usually depicted as a series of curves for differing air speeds which are expressed as a function of xe2x80x9cmachxe2x80x9d (i.e. the speed of sound). Skin waviness near the static pressure port of as little as one one-thousandth of an inch may cause a variation in the static pressure measurement sufficient to alter the altitude calculation by as much as 50 feet. Given the closeness of permissible vertical separation of aircraft in RVSM airspace even these minute variations must be closely monitored, which is why RVSM maintenance regulations call for costly and extensive maintenance procedures for the aircraft skin in the vicinity of static pressure ports.
In producing aircraft for use in RVSM air space, therefore, special care must be paid to ensuring the initial and continued accuracy of the altimeters and of the altitude readings produced thereby. It is for that reason that the FAA regulations require the availability and use of two independent altitude measurement systems at all times if an aircraft is to fly in RVSM airspace. Should one of the two error-corrected altimeters fail, the aircraft is prohibited from entering RVSM airspace and, if it fails while the aircraft is operating in RVSM airspace, the aircraft must leave that airspace, immediately.
Although some newly-manufactured aircraft have been built with a third, backup altimeter, that third altimeter is not typically certified to RVSM standards and therefore cannot be used to permit an aircraft to operate in RVSM airspace if one of the two primary altimeters fails. The costs of a third independent, RVSM-certified altimeter are simply too high to justify their inclusion as a backup unit, in that both the costs of the additional hardware itself and the need to add and maintain additional static pressure ports on the aircraft skin are so substantial.
There is accordingly a need in the art for a method and system for deploying a cost-effective backup altimeter for permitted use in RVSM airspace in the event of a failure of one of the two primary altitude measurement systems.
An RVSM-compliant altitude measuring system in accordance with the present invention includes two highly accurate primary altimeters coupled to RVSM-compliant primary static air pressure ports on the skin of an aircraft. The inventive system also includes a third, backup altimeter which is coupled to a different or separate static port. The third altimeter dynamically self-calibrates, using one or both of the primary RVSM-certified altimeters, by means of a recursive error-correcting filter (preferably a Kalman filter) continuously whenever the aircraft is in flight so that through such actual usage over time, the backup altimeter is thereby calibrated to an accuracy within RVSM mandated tolerances. Should one of the two primary altimeters fail, the backup altimeter may then replace the failed unit and thereby allow the aircraft to remain in RVSM airspace.
These and other features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.